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Researchers exposed mice to a harmless soil microbe called Mycobacterium vaccae and had the rodents perform a behavioral task commonly used to test the efficacy of antidepressant drugs.

The mice were placed in a large beaker of water for five minutes and watched to see how long they continued swimming and searching for an exit before giving up. The researchers found that the bacteria-exposed mice continued paddling around much longer than the control mice.

“At the risk of anthropomorphizing, you could say the [bacteria-exposed] mice had a more active coping style,” said study leader Chris Lowry of the University of Bristol in England.

Mice given antidepressant drugs also appear more determined to escape, Lowry added. The finding is detailed online by the journal Neuroscience.

Human test
Results from the new study are similar to those from a medical trial a few years ago in which human cancer patients treated with the bacteria reported significant increases in their quality of life.

“M. vaccae is no longer being pursued as a treatment for cancer, because it didn’t prolong life, but patients did report increases in things like vitality and cognitive function and decreases in pain,” Lowry told LiveScience. Scientists still don’t know how M. vaccae improves mood. “We don’t know the mechanism. That’s something that we would desperately like to know,” Lowry said.

The researchers suspect, however, that the microbes are affecting the brain indirectly by causing immune cells to release chemicals called cytokines.

“We know that some of these cytokines can activate the nerves that relay signals from the body to the brain,” Lowry said in a telephone interview.

Serotinin linkThe stimulated nerves cause certain neurons in the brain to release a chemical called serotonin into the prefrontal cortex, an area of the brain known to be involved in mood regulation, among other things.

“Only a very small number of neurons in the brain make serotonin, but they have massive branching projections to every part of the brain,” Lowry said.

Scientists think the lack of serotonin in the brain is thought to cause depression in people.

Previous studies have linked early childhood exposure to bacteria to protection against allergies and asthma in adulthood. The new finding take this idea, called the “hygiene hypothesis,” a step further, and suggests bacteria-exposure not only boosts our immune systems, but alters our vulnerability to conditions such as depression as well.

“These studies help us understand how the body communicates with the brain and why a healthy immune system is important for maintaining mental health,” Lowry said. “They also leave us wondering if we shouldn’t all be spending more time playing in the dirt.”